1. Field of the Invention
Exemplary embodiments of the present invention relate to a device for driving a backlight, a backlight assembly, a liquid crystal display (“LCD”) apparatus having the backlight assembly and a method for driving a backlight. More particularly, exemplary embodiments of the present invention relate to a device for driving a backlight, which uses a light emitting diode (“LED”) as a light source, having a high efficiency and a high reliability for controlling the brightness of each color light source, a backlight assembly, an LCD apparatus utilizing the backlight assembly and a method for driving a backlight.
2. Description of the Related Art
A flat panel type display device is becoming more widely used because of its relatively small size and light weight. Additionally, the flat panel type display device has an advantage in that it can realize high-resolution images.
Currently, a liquid crystal display (“LCD”) apparatus is the most widely used flat panel display device. The LCD apparatus may be defined as a display device displaying images using liquid crystal in which light transmission is changed according to an electric field. The LCD apparatus is relatively thinner and lighter than other display devices. Additionally, the LCD apparatus has a relatively lower driving voltage and a relatively lower power consumption than other display devices so that the LCD apparatus is widely used in notebook computers, mobile terminals, etc. The typical LCD apparatus includes an LCD panel assembly and a backlight assembly.
The LCD panel assembly includes an LCD panel. The LCD panel includes a thin film transistor (“TFT”) substrate, a color filter substrate facing the TFT substrate, and a liquid crystal layer interposed between the TFT substrate and the color filter substrate which changes the light transmissivity throughout the layer in response to applied electrical signals.
The backlight assembly includes a light source for generating light and optical members for improving brightness characteristics of light projected from the light source.
A cold cathode fluorescent lamp (“CCFL”) is generally used as the light source. Alternatively, a light emitting diode (“LED”) may be used as the light source. LEDs have the advantage of having superior color reproducibility compared to CCFLs.
An LED is a point light source which has a smaller light emitting area than a CCFL. Light projected from the LED light source is incident to a side portion of a light guide plate of the optical members which guides a path of the light. The light guide plate changes the path of the light exiting from the LED light source so that the light may be akin to light emitted from a surface light source, which is better suited to supply the light to the LCD panel.
Generally, in order to represent natural color, the LED light source uses a method of uniformly adjusting white chromaticity coordinates of the light emitted from the light source. It does so by controlling the brightness of three kinds of LEDs, e.g., a red LED, a green LED and a blue LED. In the above-mentioned method, the brightness of light projected from the backlight and the white chromaticity coordinates of the backlight have to be adjusted by controlling the brightness of each of the three separately colored LEDs, respectively. Generally, in order to control the brightness and therefore the white chromaticity of the LEDs, a voltage control method has been used in which a constant voltage is applied to each of the LEDs for a controlled time period.
However, the above-mentioned voltage control method applies the constant voltage to the LED via electric power supplied to the LED by modulating a pulse width of the constant voltage to control a time period for lightning the LED.
The LED in a LED light source is not constantly on. Rather, the LED blinks on and off at a rapid frequency which the human eye interprets as a constant light. Light sources utilizing LEDs may take advantage of this feature to create lights with the ability to dim or brighten. LED light sources may create the impression of dimming by increasing the time span between on and off periods and they may conversely create the appearance of brightening by decreasing the time span between on and off periods.
All LEDs have a limited lifetime, but rather than failing catastrophically as is the case in incandescent or fluorescent lighting, the LED gradually reduces the amount of light output for a given input voltage due to heating and degradation of the LED pn-junction. In order to compensate for this dimming, backlights using the voltage control method increase the pulse widths of the constant voltage supplied to the LED, thereby lighting the LED for longer periods of time and consuming more power. Essentially, to create the same brightness, the LED is turned off for shorter and shorter periods of time. The LED therefore has less time to cool between on cycles, which eventually causes the pn-junction of the LED to degrade further. The backlight using the voltage control method then must use even more power to generate even longer pulse widths and the problem compounds itself.
Another consequence of the LED light source consuming more power is that the associated heat generation effects elements of a driving board and decreases the efficiency of the driving board on which the elements are mounted. Accordingly, when the LED light source is driven by the voltage control method, a means for heat protection, such as a heat protection plate of graphite or aluminum is additionally required and thus manufacturing costs of the LCD apparatus increase.